The proposed effort will develop technology that is suited for both ultra-high resolution animal imaging systems and other high resolution applications such as human breast and head imaging. A key part of the proposed concept is the development of a highly modular approach to the detectors and supporting electronics. In this way; the basic detector/electronic modules can be assembled in a wide variety of geometries optimized for specific tasks - providing a cost effective approach to the assembly of such systems. The geometric arrangement of the modules will principally be accounted for in software rather than in the detector electronics. This approach ensures that imaging systems built using the technology can easily adapt to the changing demands of the biological research. Two types of detector modules with associated application specific integrated circuits (ASIC) are proposed for development: an ultra-high spatial resolution design using 0.8x0.8x6 mm LSO crystals and a depth-of interaction (DOI) design using 2x2x20 mm LSO crystals. The ultra-high resolution design is intended for mouse imaging applications whereas the DOI design is intended for more general purpose imaging systems. Both designs will use multi-photocathode photomultiplier tubes (PMTs) and ASICs to provide the modular design. The designs will also be easy to adapt to new technology. For example, the applicants can replace the PMTs with solid state devices such as avalanche photodiodes if they become cost effective. The effort will begin with the development of the ultra-high resolution module, related ASICs, and data acquisition system. In parallel with that effort, the basic optimization for the DOI design developed at the University of Washington will be undertaken. Once the optimization work is finished, complete DOI modules and ASICs will be fabricated. For both modules, careful characterization of the detector performance will be undertaken. Issues such as detector calibration, DOI accuracy and spatial resolution in the presence of object scatter, and count rate performance will be included. These results will be incorporated into Monte Carlo simulations covering a range of system geometries for animal imaging (e.g. mouse, rat, rabbit, dog, and primate). The final detector module designs will be fully documented (including ASIC fabrication files) to allow duplication at other laboratories.